In chronic obstructive pulmonary diseases such as chronic bronchitis, and in cystic fibrosis, the airway epithelium is constantly exposed to neutrophil elastase (NE), an inflammatory protease. The cellular response to NE dictates the balance between epithelial injury and repair. A critical factor in this balance is the activation of the epidermal growth factor receptor (EGFR), a member of the ErbB receptor tyrosine kinase family. Activation of EGFR requires homo- or heterodimerization of the receptor. A major EGFR heterodimerization partner in airway epithelial cells is ErbB2. To date, the only ligand known to activate ErbB2 is MUC4. MUC4 is a major membrane-tethered, respiratory tract mucin with epidermal growth factor (EGF)-like domains. We have made four key observations supporting a critical role for MUC4 in activating the airway epithelial response to NE: (1) NE acts on airway epithelial cells to enhance MUC4 mRNA stability and increase MUC4 protein levels. (2) NE induces tyrosine phosphorylation of EGFR. (3) NE stimulates proliferation of normal human bronchial epithelial cells in serum-free and EGF-free media. 4. MUC4, ErbB2 and EGFR colocalize in injured superficial airway epithelial cells in vivo. These observations support the hypotheses that NE triggers a molecular cascade of events in airway epithelial cells by inducing the production of MUC4, a key regulatory molecule in the cascade, activating ErbB2, and thereby promoting the heterodimerization/activation of EGFR. EGFR activation is a critical prerequisite for epithelial proliferation. The Specific Aims to be tested in this proposal are: (1) To determine whether NE induces MUC4-activation of the ErbB2/EGFR receptor tyrosine kinase cascade, resulting in epithelial cell proliferation. (2) To identify the RNA stability domains and RNA-binding proteins regulating NE-induced expression of MUC4, a key molecule in this signaling cascade. (3) To determine the relevance of these signaling pathways in mediating epithelial proliferation in vivo in chronic bronchitis and cystic fibrosis. Our ultimate goal is to use information from this project to identify new biologic targets for rational therapies to induce normal airway epithelial proliferation and differentiation following injury.